1992 Mercury 100 HP 2 Stroke Starving?

Hi Folks,

I'm posting again about a new issue. I have a 1992 Mercury 100 HP => S/N 0D129908 with low hours that's always been kept indoors mounted on it's 1991 17 foot Larson All American; except when it's on the water (fresh of course). My brother owned the boat before me, and it was his "Baby", for 18 years and he treated it like one. I too keep it in the Garage, but it's at our Family Cabin on Paul Lake, BC; and only outside on the Lake when I'm here a couple weeks a year.

Last time I had it out yesterday, I was cruising along at about 32 MPH and it shut down like it was running out of fuel. My fuel gauge was bouncing (typical) around 3/4 of a tank so I figured maybe it failed, as I haven’t put any in for a while. So last night I went to town and bought some 87 Octane Gas. (Motor requires 86.)

I filled the tank this morning but it only took 28 litres (in a 75 litre tank). I could see the fuel level looking down the fuel filler neck so it was full. Now the gas gauge was reading Full too.

So I took it for a run. Up and down to the Bluff (about half way down the lake) at 3/4 throttle was fine. So I circled again and went to the far end of the lake. No problem . I idled down and checked for fish (all deep) before getting back on plane heading towards the Bluff. I consciously decided not to go to full throttle as I thought if something happened, I’m a long way from the Cabin to paddle to it. So I ran at 3/4 throttle (about 30 MPH) with no issues. As soon as I got past the Bluff I poured the beans to it.

Of course as soon as I did that it acted like it was starving for fuel, came off plane, and shut down.

I immediately restarted it ( no problem) and got it back up on plane at 3/4 throttle. It ran for about a minute and then shut down like before.

I was able to restart it again but this time I only put it to a fast idle (about a 1000 rpm) which pushes the boat along at about 5 MPH. I then pushed it up on Plane at about 20 MPH and I made it back to the dock with no signs of distress.

I decided that trying to fix it there at the Dock was going to be painful, so I got a neighbor to assist and pulled it out.

Then my Wife and I gave it a bath, before I put it back in the garage. Remember, it is The Baby!

I usually travel here with my Laptop on which I keep an appropriate Mercury Factory Service Manual PDF File for reference. But of course this is the first time I'm trying to survive with a Chromebook instead, with virtually no documents stored. I do have a pretty generic Clymer Softcover Mercury Outboard Shop Manual 3-275 HP but there's not much of a trouble shooting guide on this sort of issue in it.

So I'm turning to you for guidance and assistance. My first guess is either the Fuel Pump diaphragm failing, or a Fuel Line that is collapsing at high flow due to age. I did have a couple lines from the Oil Injection System fail a couple years back and I replaced them - so I know the hoses are near end of life. I don't want this to become a parts cannon job, but I do plan to change the plugs, and possibly put in a new water pump that I have for it (But the one installed works just fine!). ( I put in a new Fuel Filter a while ago so I don't suspect that it is clogged)

But these are just my guesses, and I haven't yet pulled the cover off as I wanted to post here first for your input. Can you help me out?

Thanks for the assist.

Regards,

Jim

Could be a lot of things. That last 15% of the Throttle Movement, can more than double the Fuel consumption rate. Too bad you didn't do some Troubleshooting while on the Water, as you could see if anything made a Difference. Now you can only Idle the Motor, not putting it under load.
It does appear to be a Starvation Issue.
Did the Primer Bulb Collapse, at high speed?
If not, did you try squeezing the Bulb when the engine was acting up?
Could be dirt/water in the Fuel Tank, the Filter.
Could be air leaking into the Fuel Line
Look at the Plugs and see if the Ones on the Crankcases that operate the Fuel Pump are running rich
Hoses can delaminate internally, becoming Check Valves at higher Flow Rates

jimmbo said:

Could be a lot of things. That last 15% of the Throttle Movement, can more than double the Fuel consumption rate. Too bad you didn't do some Troubleshooting while on the Water, as you could see if anything made a Difference. Now you can only Idle the Motor, not putting it under load.
It does appear to be a Starvation Issue.
Did the Primer Bulb Collapse, at high speed?
If not, did you try squeezing the Bulb when the engine was acting up?
Could be dirt/water in the Fuel Tank, the Filter.
Could be air leaking into the Fuel Line
Look at the Plugs and see if the Ones on the Crankcases that operate the Fuel Pump are running rich
Hoses can delaminate internally, becoming Check Valves at higher Flow Rates

Click to expand...

I appreciate the prompt response. I know a little more in situ troubleshooting could have helped get to the diagnosis. I did not check the Bulb for collapse at the time of failure but afterwards it was fine.

While I was waiting on a response I took the cover off and started looking around. And I think I found the smoking gun. I used a screwdriver tip to point out (in the first photo) the hose that failed (it was shredded) after the zip tie clamp. (I think it silly to use Zip Ties as hose clamps on an engine.)

I wanted to trace where the other end of hose was connected, but when I touched it, it immediately failed after its Zip Tie clamp. (Photo #2)

Even though it doesn't get as Hot under the Cowl as it does under a Car Hood, those hoses can and do get brittle. I doubt the use of the Zip Tie was an Issue

They certainly are brittle.

I wish I had my Mercury Manual (PDF) with me so I could figure out exactly what the devices are, but the bottom one (at first glance) looks like part of the oil lubrication injection system. Notice the stainless hose clamp on another input hose that I replaced a couple years ago (with new hose too) that comes from the oil storage tank on the other side of the engine.

I wonder if that's the oil injection pump and it feeds into the Fuel system.

There was some oil on the inside of the cover and drips everywhere around the failure point. And a strong smell of fuel.

My latest theory is the engine protection circuit somehow detected the lack of oil injection and shut it down, or the air leak caused by the failure would not let the engine continue to run.

Tomorrow I'll dig into it a bit more. Meanwhile I'll go looking for that manual on line again.

Thanks for the inputs.

I found a Manual (not the same one as I remembered).

But from it I determined the second device is the oil pump. "B" is the inlet hose that I replaced some time ago with the SS Clamp in the 2nd photo. And "A" is the bleed screw to let the air out of the pump - which is quite visible from the partial green thread in the 2nd photo.


Now onto confirming what's in Photo #1

A little more digging and I'm pretty sure the first photo is of the Fuel Pump area and the damaged hose was connected to Check Valve F where it gets mixed with the incoming fuel before it goes into the Fuel Pump.


Hopefully, I can replace this hose and be back on the water soon. Hopefully I won't find anything new tomorrow, and I'm keeping my fingers crossed no internal damage occurred due to a lack of lubrication.

Check your stator, those were known for the high speed windings failing.

Another thing that surely should be considered is the fuel filter getting clogged with old hose bits and pieces. With the higher speeds requiring a higher fuel consumption rate, a partially blocked fuel filter could be your problem.
I don't know if your engine has a fuel filter or not....my 90 and 115 of later dates did. If you do, remove it and cut it open...if you see a lot of little black specks in it, you need a new set of fuel lines. If you see a lot of little gray specks and have a gray fuel line from the tank, that line has degraded.

On fuel or spark, you need to check the condition of the bulb when at WOT. If it's collapsed, you aren't getting fuel to the engine which could be caused by the tank vent not working, or a squeeze bulb malfunction, or a tank full of crud blocking the filter in the bottom of the tank.

Poster mentioned he had not added any fuel in some time, so...is the tank contaminated with water ? Does the owner add any fuel stabilizer to his
fuel ? No matter how the boat is stored, the rubber hoses have a limited life expectancy and need replaced at least every 10 years or sooner. Boat and motor maintenance is much more than washing and cleaning before parking in the garage.

Hi Gents,

Thanks for the tips. This engine gets very limited hours each year. I'd guess 10-20 annually. Definitely the Hoses have reached their lifespan.

The Stator was replaced a while ago and seems to be working fine. As stated earlier, I changed the fuel filter a season or two ago. I did fill the Gas Tank a month ago prior to an outing with my son. Every winter I use fuel stabilizer as appropriate.

I cleaned up the hose nipples and around the area. I've cut a replacement hose to fit and I'm heading to town to buy some Stainless Steel Clamps as I just can't bring myself to use Zip Ties. (TyRaps). I also pulled the plugs and I'm going to pick up 4 new NKG BP8H-N-10 (NKG4838 part number), and install them to the Specified Gap of 0.040"

Hopefully this repair will get The Baby back on the water.

Fuel stabilizer should be added with each filling when only being used ocassionally. Gonna bet you have some poor fuel issues. I would start with a tank of fresh treated fuel !